Hm.

I know I must be late replying to this, but I read around somewhere (think it was MTS).. Not sure though, unfortunately, mostly because I visit more Sims 3 sites than I do playing the game, make a list, play the game and verify all those questions. I am honestly not sure whether this works or not, I don't have the patience to make as I am guessing it requires a fair amount of work. :/
Continuing on the topic.. I read that if you were having multiples (LTR fertility treatment), and if you paid a sim at the top of the medical career track to tell you the gender of your baby, s/he would tell you the gender. You were then supposed to eat the fruit (apple or watermelon) opposed to the gender that s/he gave you. Then, you would have different gender kids.
Not sure if it was confirmed as it was a good week ago I read it, but it also said to try it multiple times, 'cause it might not work. :/
Awfully Affy <3